Caffeine is the most widely used drug — most of us can't make it through the day without it. But what exactly is caffeine and how does it work? The latest Reactions video from the American Chemical Society breaks it down.

When caffeine enters your body, it is broken down into three "daughter" molecules, with similar chemical structures but different actions in the body. Though they are all different, all four molecules are able to cross into the brain, where they bombard receptors on our neurons.

Caffeine specifically attaches to a receptor made for the compound adenosine, which normally causes us to relax. Caffeine blocks this "relaxation" signal, keeping us awake.

While the caffeine is lighting up the brain, its three daughter molecules have their own jobs. When caffeine enters the body, it can be broken down by the liver into three molecules: paraxanthine (80%), theobromine (10%) and theophylline (4%). These compounds can have slightly different effects than caffeine.

Paraxanthine, the most abundant caffeine metabolite in our bodies, enhances our athletic performance by helping fat break down, releasing fats into our bloodstream and fueling our muscles. It can also reduce inflammation and raise epinephrine levels in the blood, which energizes us.

This metabolite seems to have a lower toxicity and causes less anxiety than caffeine itself does. It's actually better and lasts longer when promoting wakefulness, according to a mouse study published in a paper in the 2010 issue of the journal Sleep.

Theobromine, increases the flow of oxygen and nutrients to the brain by opening up the blood vessels. This increases filtering in the kidneys, meaning it also acts as a diuretic, increasing urine production. It can also stimulate the heart.

This compound is actually found in chocolate, guarana, tea, and yerba-mate products as well, which could account for some of their energizing effects. Studies indicate that it can actually help relieve the symptoms of asthma and coughing.

Theophylline is also found in chocolate and tea and has similar effects as Theobromine. It can also help us concentrate and can have anti-inflammatory effects like Paraxanthine. It is even used as a therapy to treat respiratory diseases like COPD and asthma.

Interestingly, according to a 2008 clinical study, Theophylline could even improve a person's sense of smell.

While that morning cup of coffee may bring good tidings, the American Chemical Society cautions that it's best to keep your caffeine intake at about 400mgs, lest you end up with jitters and anxiety. That's three eight-ounce coffees, five eight-ounce red bulls, or eight cups of black tea.

Some people are more sensitive to caffeine than others, but for the average person the compound becomes toxic when someone ingests about 10 grams. That is 75 cups of coffee.